1. Field of the Invention
This invention relates to integrated circuit manufacturing and more particularly to the patterning of layers of materials formed upon surfaces of semiconductor substrates.
2. Description of the Relevant Art
Integrated circuit features are typically patterned from a layer of a desired material formed upon a surface of a semiconductor substrate by removing unwanted portions of the layer. This process is called patterning, and is typically carried out using a process called photolithography. In photolithography, a layer of a light-sensitive material is first deposited on the surface of a layer to be patterned. The light-sensitive material undergoes a physical or chemical change when exposed to light. Such light-sensitive materials function as etch barriers, and are called resist etch barriers or simply "resists". Molecules of a "positive" resist material chemically join together (i.e., polymerize) in the absence of light. Exposing a portion of a positive resist layer to light causes the molecules therein to break apart, converting the exposed portion to a more soluble state. Molecules of a "negative" resist material, on the other hand, polymerize only when exposed to light. Select portions of the resist layer are exposed to light passed through a pattern (i.e., a mask). The polymerized portions of the resist layer are able to resist an etchant during a subsequent etching step.
Following exposure, the resist layer is subjected to a chemical which dissolves the unpolymerized portions during a "developing" step. The developing step forms voids in the resist layer. The remaining portion of the resist layer protects the underlying portion of the layer to be patterned during the subsequent etching step. During the etching step, the portion of the layer to be patterned not covered by the remaining portion of the resist layer is chemically removed by an etchant. Following the etching step, the remaining portion of the resist layer is removed, leaving the desired features on the surface of the semiconductor substrate.
The limit to the smallest physical dimension of a feature which may be formed using photolithography is dependent upon the wavelength of light used. Shorter wavelengths allow the formations of features having smaller physical dimensions. Most photolithography is currently performed using ultraviolet light having wavelengths between 350 and 450 nanometers (nm). The trend in photolithography is toward shorter and shorter wavelengths of light.
Several alternate lithography techniques offer the ability to form features having physical dimensions smaller than those obtainable using photolithography, including x-ray lithography, electron beam lithography, and ion beam lithography. Like photolithography, all three of these alternative lithography techniques use energy to transfer a pattern to a resist layer. X-ray lithography uses x-rays of very short wavelength (e.g., 40-500 nm) to transfer a pattern to a layer of a material having a high sensitivity to x-rays (i.e., an x-ray resist layer). Electron beam lithography uses electrons, having wave-like properties, at energy levels equating to wavelengths between 2 and 5 nm. The electrons are formed into a beam, and the beam is scanned across the surface of a layer of a material which undergoes chemical or physical changes upon exposure to the electron beam (i.e., an electron beam resist layer). In ion beam lithography, ions (i.e., charged atoms) are used to transfer a pattern to a layer of a material having a high sensitivity to ions (i.e., an ion beam resist layer). The above alternative lithography techniques have not yet matured, however, and are currently slow and expensive when compared to photolithography.
It would be beneficial to have a lithography process capable of forming features having physical dimensions smaller than those obtainable using photolithography and at least comparable to photolithography in terms of speed and cost. Such a lithography process would represent an advancement in wafer fabrication technology.